1. Field of the Invention
The present invention relates to varactor-type tuning systems for television receivers and in particular to a summing circuit therefor, for summing an AFT signal and a tuning voltage.
2. Description of the Prior Art
Varactor-type tuning circuits are well-known in the art and particularly have been adopted for tuning tuners including both UHF and VHF sections of a television receiver. Such tuning circuits incorporate a varactor diode whose capacitance is dependent upon a tuning voltage applied thereto. The amplitude of the tuning voltage is determined selectively to set the channel to which the tuner is to be tuned. Further, it is well-known in the art to affect or adjust the tuning voltage as applied to the varactor-type tuning system by adding thereto an AFC or AFT correction signal. Typically, an AFT correction signal is obtained from a frequency-sensing circuit within an IF amplifier of the television receiver. In turn, the AFT correction signal is applied to the local oscillator circuit of the varactor-type tuner and in particular to a voltage-dependent reactive element, whose characteristics are varied to change the output of the local oscillator. The sensed polarity of the correction voltage is such that if the IF frequency is not correct, the frequency of the local oscillator is adjusted in a direction to tend to minimize the error or correction voltage. In prior art tuning systems, the reactive element has been an active element or, as in varactor-type tuning systems, the reactive element takes the form of a semiconductor device having a reverse bias junction, the capacitance of which varies inversely with the amplitude of the applied voltage; these devices are known as "varactors".
The use of varactors complements the operation of an automatic fine-tuning (AFT) circuit, in that the AFT correction signal is added to the primary source of tuning voltage in a manner such that the AFT correction signal is switched on or off without disturbing or altering the primary tuning voltage. To this end, it is a common technique to "float" the frequency-sensing circuit of the IF amplifier, so that it may be connected in-series with the source of the primary tuning voltage. However, this is not possible in those tuning systems implemented with integrated circuitry in that the AFT circuit is disposed within the integrated circuit and must be referenced to a common ground. It has been suggested that where such circuits are implemented by semiconductor integrated techniques, the output of the AFT circuit be adapted to provide a differential output, i.e. a two-terminal output with a DC offset voltage common to both output terminals. The offset voltage may be canceled by injecting a DC voltage equal to the offset voltage.
Exemplary of the prior art AFT summing circuits is U.S. Pat. No. 3,878,466, which discloses that the output of its AFC or AFT circuit is amplified by a pair of transistors and applied to a resistive-type network in the form of a bridge, wherein a first input is an amplified AFT circuit output and is applied to a first terminal of the bridge network, while the tuning voltage is applied to a diagonally-opposed terminal of the bridge network, with the summed output taken from the other two terminals and applied, respectively, to the VHF and UHF tuners. Experience has shown that the use of employing a resistive network for coupling and summing the tuning voltage and the AFT correction signal is that the output of the AFC or AFT circuit cannot be disconnected or adjusted for the particular tuning range, i.e. VHF or UHF, that is being received by the television receiver. In this regard, the impedance of the resistive network must be high, in order that the presence or absence of the output of the AFT circuit will not affect the tuning voltage. However, if the impedance of the resistive network is selected to be high, the relative effect of the output of the AFT circuit becomes small. For example, if the tuning voltage varies in the range of 20-30V and a relatively high resistive network is used, the relative change for such tuning voltage becomes increasingly smaller, with correspondingly less effect upon the local oscillator.
Further, it is noted that a typical UHF varactor tuner is very sensitive to the presence of an AFC correction signal and it is desired that when the UHF varactor tuner is operative and the television receiver is receiving UHF signals, the output of the AFT circuit be attenuated. In this regard, prior AFT summing circuits have not satisfactorily accounted for either the attenuation or the disconnecting of the output of the AFT circuit, without unduly affecting the tuning voltage.